First in a debate Tuesday and now in a television ad that began airing statewide Thursday, Gomez has been pitching the relative brevity of the term he is trying to win over Democratic nominee Edward J. Markey. He describes it as a probation period of sorts, as he finishes out the term for the seat vacated by John F. Kerry when he was appointed secretary of state.

“You’ve had 37 years in D.C. to get these important things done,” Gomez told Markey during their final debate, in a segment repackaged for the 30-second ad. “Give me 17 months, and I will keep my word, and I will do what I say.”

Gomez is essentially offering voters a recall mechanism: Don’t like how I’m doing my job? In 17 months, vote me out. Think I’m too cozy with the Tea Party or that I’ve become a loyal foot soldier for the far right? Two Novembers from now, pick the other guy.

Pat Greenhouse/Globe staff

After appearing at a Canton press conference Thursday with Norfolk County Sheriff Michael G. Bellotti and retired teacher Sue Leahy, US Senate candidate Edward J. Markey headed off to another campaign stop. The election is next Tuesday, June 25.

Gomez campaign aides insisted that the private equity investor conceived the line himself in between campaign events, after months of hammering Markey for his long service in Congress.

“He was trying to think of a way to add onto the ‘37 years’ argument,” said Gomez spokesman Will Ritter. “He had some time to prepare for the debate, and he came up with it.”

‘The guy seems like he has nothing to recommend himself to voters.’ — Larry Rasky, Longtime adviser to Democratic nominee Edward J. Markey

After Gomez hit Markey with the line during the debate, the campaign’s media and ad teams worked through the night to prepare spots for the web and television, Ritter said.

“As the debate was going on, we knew that we had something,” he said.

The 11th-hour pitch allows Gomez to make a reassuring argument to voters, rebutting Markey’s effort to link Gomez to conservative forces within the GOP. With Gomez personally offering a sunset option for his time in the Senate, the campaign is betting, voters will view him as less of a risk.

It could also be Gomez’s last chance to swing undecided voters. By confining his tenure to a prescribed time frame, it minimizes the consequences of the choice before voters. But in a state where Senate seats, until recently, could stretch generations without changing hands, the notion of such a fleeting stint could be jarring to voters.

In a press release, Markey’s campaign tried to paint a 17-month Gomez term as a year-and-a-half of opportunity to join with the national GOP, markedly to the right of much of the Bay State electorate.

“The guy seems like he has nothing to recommend himself to voters,” Rasky said, mocking Gomez’s sales pitch as “not only have I never hit a curveball when I played in the Pony League, now I’m asking you to send me to hit Major League pitching.”

Gomez has struggled to whittle Markey’s solid lead in the polls, and Democratic Party leaders are gearing up for what some call an historic special election get-out-the-vote operation, one that the smaller state Republican Party will probably be unable to match.

Gomez adviser Lenny Alcivar said the line was organic. “Don’t overestimate the thinking behind this,” Alcivar said. “This isn’t poll-tested. This is an extension of everything we’ve been saying.”

In his first general-election ad, Gomez spoke Spanish and discussed his Navy SEAL service, before reciting some of the Pledge of Allegiance. The second ad, also heavy on biography, showed footage from his primary-night victory party and closed on a similarly familiar note, with Gomez saying, “This election’s about the future; it’s not about the past.”

Since the 17-months argument is not a complicated one and appears to offer the twofold benefit of zinging Markey while at the same time ameliorating voter concerns about an untested politician, Ritter was asked why it took so long to develop the message.